Method of obtaining electron beam of controlled low energy



March 6, 1956 R. E. HONIG 2,737,606

METHOD OF OBTAINING ELECTRON BEAM 0F CONTROLLED LOW ENERGY Filed Dec.12, 1951 2 Sheets-Sheet 1 INVEN TOR. 2704220 5. //&/V/6

March 6, 1956 HONIG 2,737,606

METHOD OF OBTAINING ELECTRON BEAM OF CONTROLLED LOW ENERGY 2Sheets-Sheet 2 Filed Dec. 12, 1951 i/cA/fleo E. Na ca BY United StatesPatent METHOD OF OBTAINING ELECTRON BEAM OF CONTROLLED LOW ENERGYRichard E. Honig, Princeton, N. J., assignor to Socony Mobil OilCompany, Inc., a corporation of New York Application December 12, 1951,Serial No. 261,337

4 Claims. (Cl. 313-78) In many applications of electron physics, theproduction of mono-energetic electron beams of well-defined, lowenergies is of importance. All sources of electron beams presently knownproduce particles with initial energies ranging from zero to severalelectron volts, the distribution function often being Maxwellian. Forexample, of the electrons emitted by a tungsten filament run at 2500 K.,have initial energies in excess of 0.5 volt. Thus, in experimental workusing low-energy electrons, such as the study of chemical reactionsinitiated by controlled electrons, the determination of criticalpotentials, the application of critical potentials to experimental andanalytical processes, and the like, the thermal energies are of amagnitude too large to be neglected.

To reduce the distribution of thermal energies, energy filters have beenused in the past that are based upon electric or magnetic fields, or ona combination of the two. In particular, the followingschemes haveproven useful:

1. Transverse magnetic filter-480 2. Radial electrostatic filter-127 3.High frequency, plane electrodynamic filter 4. Crossed transverseelectrostatic and magnetic filter.

Of these four schemes, the first is the simplest and possibly the onemost used. It is widely used in such instruments as the massspectrometer. However, all of these are subject to one consideration. Inall, the earths magnetic field is present and acting and must beconsidered and controlled, eliminated, or overcome. The transversemagnetic filter type of operation requires an extended, homogeneousmagnetic field of constant magnitude. To attempt to realize this byartificial means, at proper intensities for the production of electronbeams of tightly controlled low energy and at the same time eliminate,control, or erase the eitects of the earths magnetic field is difiicult.

This invention is based upon the realization that the earths magneticfield is homogeneous over a practically limitless volume, that it isconstant to within about 0.1% of its value over time periods relativelyextended with respect to periods in which it will normally be utilized,and that its intensity is just right for the banling and analysis ofelectrons of a few volts of energy. This invention then, as may be seenfrom the drawings attached to and made a part of this specification, isessentially an electron analyzer in which the earths magnetic field isused as a transverse magnetic filter, the source, path and slit elementsof which are so mounted that they are capable of being brought intoproper relationship with'the geomagnetic field at the point and time ofoperation. In these drawings, Figure 1 is a schematic drawing of theinstrument setup, and Figure 2 is a similarly schematic showing of amount for same. Figure 3 shows a schematic electrical set-up of theinstrument.

Turning now to Figure 1 of the drawings, 10 is a justable slit.

2,737,606 Patented Mar. 6, 1956 filamentary source of electrons and 11is a shell electrode having a directional slit 12 through whichelectrons are emitted in the desired direction. Split plate 13 serves toadmit these electrons to the curved path portion of the apparatus,housed in tube 14, in which the electrons describe a path of radius R,the magnitude of which is dependent upon the desired energy of electronsto be selected and upon the magnitude of the magnetic field F, which isthe terrestrial magnetic field existingat the point and time ofoperation of the apparatus, the apparatus being so mounted as to becapable of positioning to bring the plane of the electron path normal tothe direction of the geomagnetic field. At 15 we find a second slitplate and at 16 an electronically ad- The path R is housed in a metaltube, electrically integral with slits 13 and 15, providing anelectrostatic shield, necessary to prevent the charge-up of glass wallswith stray electrons. Vessel 17 serves to house the electron source andvessel 13 to house the process, instrument, or whatever item may bedesired for a target of the selected electrons. Connections are providedat 19, 20, 21 for pumping out the various portions of the apparatus tothe desired vacuum level, and it will be understood that whateverelectrical leads, devices for reaction, targets, and the like aredesired may be placed in vessel 18.

In Figure 2, highly schematic, it is shown how the device may be mountedto be placed in desired relation to the geomagnetic field. In its mostsimple form it could be a support 22 for the device itself, mounted upondouble swivels 23 and 24. In an elaborate form, could be a servo-motordriven device of the character of an equatorial telescope mounting,electronically controlled to be held accurately in the geomagneticfield, although the magnitude to time-direction variations of thegeomagnetic field is ordinarily quite small.

Figure 3 shows a schematic electrical setup for the instrument, and tothe right of it in the same figure, an ion collector and an electrontrap which might be used in case the electrons produced were to be usedto ionize a gas contained in vessel 18. In this figure, the same membersare used for the parts found in Figure 1. This figure is largelyself-explanatory. In Figure 3, electrons are generated at 10, 11,passing out through orifice 12 and thence through lens orifices in 13,15, and 16 at opposite extremities of the collimating tube 14, in whichtube the electron is exposed only to the influence of the geomagneticfield. The emergent electrons, acting to ionize by bombardment a gaswhich is passed through vessel 18 from entry 19 to exit 20, finally endtheir travel in electron trap 21. The ionization resulting from thisbombardment of gas confined in 18 is detected by ion collector 22.

It will be understood, of course, that this instrument should beshielded from magnetic fields other than geomagnetic, and that it isadvisable, when it is in operation, to have datum plane observation ofthe time variations of the geomagnetic field. These features willpartake of the nature of the shielding and base datum observationalready developed and widely used in connection with magnetometers, usedfor oil prospecting by observations of variations in the geomagneticfield.

I claim:

1. Apparatus for producing a beam of monoenergetic low energy electronscomprising a source of electrons, collimating means to form a directedbeam from at least some of the electrons emitted thereby, means tosubject the beam therefrom to dispersion by the geomagnetic field in thesubstantial absence of other dispersion force, and a second collimatingmeans to produce a monoenergetic electron beam from a selected portionof the dispersed electrons, said second collimating means be so locatedas to collimate only that portion of the electron dispersion spectrumcomprising electrons of the desired energy.

2. Apparatus for producing a beam of monoenergetic low energy electronscomprising a source of electrons, collimating means to form a directedbeam from at least some of the electrons emitted thereby, means fordirecting the longitudinal axis of said collimating means into a planenormal to the geomagnetic field to permit dispersion of the electronsthereby, and a second collimating means to produce a monoenergeticelectron beam from a selected portion of the dispersed electrons, saidsecond collimating means be so located as to collimate only that portionof the electron dispersion. spectrum comprising electrons of the desiredenergy.

3. Apparatus for producing a beam of monoenergetic low energy electronscomprising a source of electrons, collimating means to form a directedbeam from at least some of the electrons emitted thereby, means fordirecting the longitudinal axis of said collimating means into a planenormal to the geomagnetic field to permit dispersion of the electronsthereby in accordance with their energies, and means to select andcollimate dispersed electrons whose energies fall Within. a desiredrange to form therefrom a monoenergetic electron beam of low energy.

4. Apparatus for producing a beam of monoenergetic low energy electronscomprising a source of electrons, collimating means to form a directedbeam from at least some of the electrons emitted thereby, means fordirecting the longitudinal axis of said collimating means into a planenormal to the geomagnetic field and maintaining it in such position topermit dispersion of the electrons thereby in accordance with theirenergies, and a second collimating means to produce a monoenergeticelectron beam from a selected portion of the dispersed electronscomprising only electrons of the desired energy level.

References Cited in the file of this patent UNITED STATES PATENTS1,895,373 Bruche Jan. 24, 1933 1,977,615 Beers Oct. 23, 1934 2,025,580Engelhardt Dec. 24, 1935 2,027,393 McCreary Jan. 14, 1936 2,103,507ZWorykin Dec. 28, 1937 2,114,283 Anderson Apr. 19, 1938 2,260,041 Mahlet al. Oct. 21, 194l 2,441,269 Hartig May 11, 1948 2,563,333 Herzog Aug.7, 1951 2,587,481 Kaehni et al Feb. 26, 1952

